The militaryâ€™s reusable space plane, the X-37B, and its classified payload lifted off in April only one day after the maiden flight of the Defense Advanced Research Projects Agencyâ€™s Hypersonic Technology Vehicle-2 suborbital glider. It flew nine minutes before operators lost its signal and were forced to abort the mission.

These two vehicles, along with a hypersonic missile that made its first test flight one month later, the X-51 WaveRider, have all been mentioned as means to carry out the â€œprompt global strikeâ€ concept, which calls for the U.S military to deliver a conventional warhead anywhere on the planet in significantly shorter time spans than are currently possible.

Intelligence can be fleeting. The location of a high-value target such as a terrorist leader can be confirmed, but he may move before an air strike is arranged. Or he could be located in a nation that doesnâ€™t allow the Air Force to fly over its territory. U.S. Strategic Command has been looking into ways to deliver bombs on such targets for several years.

The X-37Bâ€™s top-secret payload has nonproliferation experts wondering if it is meant to deliver weapons. Air Force leaders wonâ€™t say how high up the experimental spacecraft is, how much it costs, or exactly how long it will loiter before returning to Earth. And it definitely wonâ€™t reveal what it is carrying in its bay.